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bradnree
12-31-2006, 02:31 PM
At what compression is a cylinder deemed too low and ineffective ?? What are the causes of low compression ??........Brad

PackardV8
12-31-2006, 03:03 PM
Any time the cold cranking compression is 25% lower than the spec, there is major mechanical trouble or wear. The classic test is to apply compressed air in each cylinder with the piston at TDC on compression stroke and listening at the carb, oil filler and exhaust.

1. Burned valves - intake will be heard in the carb, exhaust at the tailpipe. Usually only one or two cylinders affected.
2. Worn piston rings will leak compression into the oil pan and be heard at the oil filler. Needs to be at TDC. Usually all cylinders are worn about the same
3. Head gasket

thnx, jv.

PackardV8

dictator27
12-31-2006, 03:19 PM
Another possible cause for low compression readings is a bad intake manifold gasket. On a V8 a bad gasket will result in identical low readings in diagonally opposite cylinders in the engine. For example front cylinder on drivers side, rear cylinder passengers side.

Terry Godkin
27 Dictator sedan
54 Commander Starliner
Surrey British Columbia

bradnree
12-31-2006, 04:23 PM
I have the compressed air but what fitting replaces the spark plug for this test ??........Brad

PackardV8
12-31-2006, 05:38 PM
Hi, dictator27,

Can you help me understand why a "bad" (assume leaking?) intake manifold gasket would cause a low compression reading? Since compression testing is always done with the throttle wide open, there is no vacuum in the intake manifold, so how would the cylinder know the intake gasket was leaking?

bradnree, you can buy the fitting, but I made my own by knocking the ceramic out of an old plug and tapping it for a standard male thread air hose quick disconnect fitting.

thnx, jv.

PackardV8

John Kirchhoff
12-31-2006, 05:44 PM
Some compression testers have a removable end you can adapt to the air hose, otherwise you can break the porcelain out of an old sparak plug and braze a small piece of pipe to it and screw a male air chuck fitting to that. The porcelain is pretty tough stuff to get out but I've made several of these for different engines.

Speaking of low compression, many years ago my dad bought a new compression gauge and tried it out on his father-in-laws worn out '38 Chevy car. The first three cylinders showed reading at all and he finally got 25 and 30 pounds on two cylinders. Thinking the gauge was faulty, he checked his car and the gauge was just fine. Funny thing was the "no compression on four cylinders" car always started no matter how cold it was and got grandma and grandpa there and back.

dictator27
01-02-2007, 07:27 PM
Hey Packard V8

You are absolutely right of course. I was obviously suffering from a major case of brain freeze and relying on amemory which proved to be less than perfect. [:o)][B)]

Terry Godkin